The present invention concerns a fuel injector.
A fuel injector is discussed in International Patent Application No. WO895/0478. The damping device discussed in this document is made of a pot-shaped damping element, a weak compression spring having a low spring coefficient and a strong compression spring having a high spring coefficient. The two compression springs are staggered axially relative to each other and surround the valve needle by sections. The pot-shaped damping element is situated between the two compression springs which act on the pot-shaped damping element in opposite direction and which, on the side facing away from the pot-shaped damping element, are in each case braced against support elements applied to the valve needle. The weak compression spring counteracts the closing of the fuel injector, the strong compression spring counteracts the opening of the fuel injector. Formed between the edge of the damping element and the inner wall of the valve housing is a narrow, circumferential gap, extending in the axial direction, which is filled with fuel. Therefore, in response to movement of the damping element, a shear force develops in the fuel liquid between the edge of the damping element and the inner wall of the valve housing, the shear force producing a frictional force which counteracts the movement of the damping element. In cooperation with the compression springs, damping of the valve needle is thus achieved.
The known fuel injector has the following disadvantages: The damping force is permanently predefined by the spring force and the shear force, and therefore cannot adapt to the performance quantities of the internal combustion engine; in particular, it is not variably adjustable as a function of time. Since the fuel inflow in the direction of the sealing seat is influenced by the damping plate, flow eddies occur in the fuel, thus causing the moldability of the fuel discharge to deteriorate. A fuel inlet below the damping plate, suggested in WO 89/10478 as an alternative, is believed to be impractical, since it may markedly increase the size of the valve housing on the discharge side. Furthermore, due to the additional mechanical components, the fuel injector is more susceptible to wear, especially since the damping force is dependent on the width of the gap formed between the edge of the damping element and the inner wall of the valve housing.
In U.S. Pat. No. 5,236,173 is discussed a clamping spring in the form of a disc spring between the valve-seat member and a valve-seat support on which the valve-seat member is mounted, so that the valve-closure member strikes gently against the valve-seat surface formed on the valve-seat member. However, the disadvantage of this type of damping is that, after the valve-closure member has struck, the valve-seat member swings through in the spray direction, while the valve-closure member either comes to a standstill, or, because of the reversal in impetus, moves back from the valve-seat member contrary to the spray direction. For this reason, valve bouncing can even occur in increased measure with this fuel injector design, so that this type of damping has not proven to be very worthwhile.
The fuel injector of an exemplary embodiment of the present invention is believed to have the advantage that the fuel injector is debounced in a satisfactory manner. In addition, the electromagnetic damping device requires no mechanically stressed components such as compression springs and disc springs, and needs no damping fluid. Furthermore, the damping device is temperature-stable and permits a variable damping force.
The damping device advantageously has an excitation coil for generating a magnetic field, and at least one electroconductive induction loop arranged on the valve needle. In this manner, the electromagnetic field necessary for the damping can be generated in a simple manner. In addition, the damping force can act directly on the valve needle.
It is advantageous that the excitation coil is wound onto a valve housing of the fuel injector, the valve housing having a circumferential groove for this purpose. This results in an accommodation of the excitation coil which is simple from a standpoint of production engineering, and in which the excitation coil is well protected and can easily be replaced.
Another advantage is that the electric conductivity of the induction loop is greater than the electric conductivity of the valve needle. A loop voltage induced in the induction loop thereby produces an electrical induction current conducted in the induction loop.
It is also advantageous that the induction loop is electrically insulated from the valve needle. The electromotive force is thus particularly well utilized.
A further advantage is that the induction loop is sleeve-shaped and surrounds the valve needle by sections. This results in an induction-loop form which is adapted to the geometry of the injector valve, and which also permits simple mounting on the valve needle.
The axial length of the induction loop along the valve axis is advantageously less than the axial length of the excitation coil along the valve axis. A greater loop voltage is thereby induced in the sleeve.
A control unit advantageously has a current regulation for the current-regulated driving of the excitation coil and/or the actuator. This permits an exact, quickly reacting control of the damping force acting on the valve needle.
To utilize the displacement current developing in response to the compression of the actuator, the excitation coil is advantageously connected in series to the actuator. Thus, the energy stored in the actuator can be used for damping the valve needle.